Vapor generating and superheating unit



May 1962 P. H. KOCH ETAL' 3,033,177

VAPOR GENERATING AND SUPERHEATING UNIT Filed July 2, 1956 6 Sheets-Sheet 2 IN V EN TORS W ATTORNEY May 8, 1962 P. H. KOCH ETAL VAPOR GENERATING AND SUPERHEATING UNIT 6 Sheets-Sheet 3 Filed July 2, 1956 INVENTORS Pau/ 1. X0012 BY J rflzur Jfluy/zes FIG.3

May 8, 1962 P. H. KOCH ETAL 3,033,177

VAPOR GENERATING AND SUPERHEATING UNIT Filed July 2, 1956 6 SheetsSheet 4 F I G 4 INVENTORS ATTORNEY y 1962 P. H. KOCH ETAL 3,033,177

VAPOR GENERATING AND SUPERHEATING UNIT Filed July 2, 1956 6 Sheets-Sheet 5 65 \vnxme HEADER J F T ECONOMIZER J C UPPER ENCLOSURE 6 WALLS J J F C C 6 3%? I TOTSUUPPSORT 68 H 5%) BE I C J J T SECONDARY SUPERHEATER ATTEMPERATOR 645 How \LOWER ENCLOSURE WALLS L L TElIZEsRlNG u U CYCLONE WNAM FURNACES RADIANT LOWER WATER WALLS IN VEN TORS ATTORNEY May 8, 1962 EH. KOCH ETAL 3,033,177

VAPOR GENERATING AND SUPERHEATING UNIT Filed July 2, 1956 6 Sheets-Sheet 6 FlG.6 2

FIG. 7

62 i INVENTORS .Pmz/ Jq JQch flrzfiur Jfluybes ATTORNEY United States Patent Ofitice Patented May 8, 1962 3,033,177 VAPOR GENERATING AND SUPER- HEATING UNIT Paul H. Koch, Bernardsville, and Arthur 1. Hughes, Paul:-

anack Lake, NJ., assignors to The Babcock & Wilcox Company, New York, N.Y., a corporation of New Jersey Filed July 2, 1056, Ser. No. 595,163 8 Claims. (Cl. 122235) This invention relates in general to vapor generating units, and, more particularly, to forced flow oncethrough steam generating units.

Forced flow once-through steam generating units have been operating in Europe for a number of years and most of these units have experienced difficulty of operation due to corrosion and/or deposition of water-carried solids in the section of the unit where the liquid is finally converted to steam. Such a section is usually designated the transition zone. The European units have special provisions for withstanding such corrosion and/or deposition and under their operating circumstances they have been economically justifiable. However, in this country such units have never been constructed in large commercial sizes, such as used by the utilities for the generation of electricity. The U.S. operators have heretofore considered that the problems associated with such boilers could not be economically justified in this country. However, in the recent past there has been the renewed interest in steam generators of the once-through type, and particularly, in those which would operate above the critical pressure of 3206 p.s.i.a. Under such conditions the problem of corrosion and/ or deposition is not of the same character as previously encountered because above the critical pressure the fluid changes from liquid to a vapor rather suddenly and with no change in density and during which change there is no boiling to induce corrosion or deposition.

It has been previously proposed that once-through steam generators be operated with the transition zone occurring in the low temperature convection portion of the unit so that corrosion :and/ or deposition therein will cause a minimum of damage and result in longer life of the unit.

The present invention provides a commercial size forced circulation once-through vapor generating unit characterized by the transition zone and initial superheating. in the radiantly heated enclosure walls of the setting and provides a means for introducing recirculated gas into the furnace chamber of the unit to maintain a relatively low heat input rate to the transition zone.

More specifically, the invention provides a steam generating unit having a furnace chamber arrangement having an upper and lower portion, ,where slag forming fuel is burned in the lower portion wherein the majority of the'ash' is collectedas molten slag and is discharged; The upper furnace portion is divergently expanded from the lower portion in a symmetrical manner to provide a chamber for reducing the temperature of the gases and collect slag in a dry form prior to'the gas entry into a superjacent convection'chamber within the setting.

Additionally, the invention contemplates an arrangement of cyclone type furnaces arranged on opposite sides of the furnace to discharge into an unobstructed small volume slag collecting chamber and gas recirculation means arranged to discharge cool recirculated products of combustion at theexit from said small volume furnace to'control the resulting furnace gas temperature, such that predominantly all the slag in said furnace gases has solidified prior to entry into .theconvection passes.

Additionally, there are provided tubular fluid heated supports for carrying a portion of the loading of the setting from upper structural support members.

The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawings and descriptive matter in which there is illustrated and described a preferred embodiment of the invention.

Of the drawings:

FIG. 1' is a partial diagrammatic sectional elevation of a forced circulation once-through steam generator for operation at supercritical pressure in accordance with the present invention.

FIG. 2 is a partially diagrammatic plan section taken on the line 22 of FIG. 1.

FIG. 3 is a partial plan section (with a portion broken away) of the gas recirculation arrangement taken on the line 33 of FIG. 1. 4

FIG. 4 is a plan section taken on the line 4-4 of FIG. 1.

FIG. 5 is a diagrammatic representation of the vaporizable fluid flow arrangement within the steam generator of FIG. 1.

FIG. 6 is a partial vertical section of the details of the cyclone furnace supports; and

FIG. 7 is a fragmentary view on an enlarged scale showing the manner of using the tubular supports of In the drawings there is illustrated the invention as embodied in a forced flow once-through steam generating unit intended for central station use. This particular unit is designed for a maximum continuous steam flow of 2,900,000 lbs. per hour at a pressure of 3625 p.s.i.g. and a total steam temperature of 1050 F. at the superheater outlet based on feed water being supplied at 4500 p.s.i.g. and with coal firing. The unit includes two steam reheaters, one to raise the temperature of 2,5 30,000 lbs. of steam per hour at 1,300 p.s.i.g. from 787 F. to 1050 F. and the second to raise 1,980,000 lbs. of steam per hour at 330 p.s.i.g. from 750 F. to 1050 F.

With particular reference to FIG. 1 there is shown a vertically extending-setting of rectangular cross section providing a furnace chamber A and asuperjacentconvection gas cooling chamber B. The setting is completely lined with fluid heating tubes from the lowermost portion ofA to the uppermost walls of B; The unit isarranged with eight cyclone furnaces 10' which are independently fired by crushed or granulated coal and are of the general character of U.S. Patent No. 2,357,301. Four of the cyclones are arranged to separately discharge combustion products and molten slag throughthe wall 12. into a small volume portion 14 of the furnace chamber A. The other four cyclones are oppositely arranged to separately discharge combustion products and molten slag into the portion 14 of the furnace chamber A through the opposite wall 16. End walls 18 and 20 bound the setting from top to bottom, each in a single vertical plane. Thus the walls 12, 16, 18 and 20' form a small volume furnace 14 having slag discharge openings 21 in the lower portion thereof to discharge molten slag received from the cyclones 10 into a lower slag tank 21a. The lower portion furnace chamber walls 12, 16, 18 and 20 are shown diagrammatically but, in practice, they include a multiplicity of. small diameter fluid heating tubes covered by refractory to reduce the heat input thereto and to maintain the temperature. in the cyclonesand in V wardly uniformly and symmetrically to present sloping walls 22, 23 and thence rise vertically upward as enclosure walls 24, 25 which in conjunction with end walls 18 and 20 form the enclosure walls of the setting including the upper furnace portion 26.

The convection chamber B is separated from the upper furnace portion 26 by the tube screens 27 and 27a and which are formed by certain tubes taken from walls 24 and 25. These tubes rise vertically upward in spaced relationship to support a secondary superheater 28 which spans the entire setting width in the lower portion of the convection chamber B. Above the superheater 28 the vertical tubes form fluid tight baffles 29 and 29a to define three parallel gas passes 30, 31 and 32.

A primary superheater 35 of the unit is arranged in the pass 32 while the first reheater 36 is arranged in the central pass 31 and the second reheater 37 is arranged in the pass 30. Above the superheater and reheaters, and in each of the gas passes 36, 31, 32 an economizer is disposed. 38, 39, 40, one in each pass. Gas dampers 41, 42, and 43 individually control the gas flow rate in each of the gas passes 30, 31 and 32, respectively. At the very top of the unit there is a gas breeching 44 arranged to direct the gas upwardly through the air heater 45 and thence through the precipitator 46 to the stack (not shown).

A gas recirculation fan 47 is arranged to remove a portion of the gas from the breeching 44 via a duct 48 and to discharge the gases into supply ducts 49 and 49a (FIG. 4) which are on the opposite end walls 13 and 2d of the setting. The gases pass downwardly and into dis tributing chambers 50 and 51 which run the full width of the setting above the cyclone furnaces and are arranged to receive gas from the supply ducts 49 and 49a. A multiplicity of gas openings or nozzles (preferably dampered) 52 are in the opposite parallel walls 12 and 16 and are uniformly distributed across the entire width thereof to provide recirculated gas arrangements which mixes recirculated gas with the combustion products from the lower portion 14 of the furnace. This controls the gas temperature entering the convection chamber B to a value below the ash fusion temperature. The gas recirculation rate in the unit at maximum continuous rating is approximately 40 percent of the gases generated by the combustion of the fuel. In addition, to controlling the slagging temperature of the combustion gases, the recirculated gas effects a lower heat input rate to the furnace walls 18, 20, 24 and 25 of the upper portion of the furnace A.

Combustion air for the cyclone furnaces is supplied by the forced draft fan 53 through the air heater 45 and down the individual supply ducts 54 which are symmetrically arranged along the length of the setting and which supply each cyclone individually. Within each of these supply ducts there is a venturi section 55 arranged to meter the air for the control of combustion. Adjacent the air ducts a portion of the coal hoppers 56 are shown and arranged to supply crushed coal to the cyclones 10.

The steam generator setting is supported by structural steel members having upright members 57, upper cross beam 58 and lower cross beam 59. These structural members are of suflicient size and strength to top support the entire load of the steam generator. A portion of the load of the steam generator is supported by a multiplicity of tubular fluid heated support tubes 6% which are connected to upper cross beam 59 and extend downwardly to carry the load of a plurality of lower cross beams 61. These tubular supports 60 and cross beams 59 and 61 are arranged in such a manner that there is a multiplicity of these beams located at spaced positions across the width of the unit. The tubular elements are connected at top and bottom to the headers 62, 63 and It is divided into three sections 64. This support arrangement will be more completely described hereinafter.

In the operation of the steam generating unit described, relatively coarse crushed coal is independently and controllably delivered to the separate cyclone furnaces 10 wherein the fuel is burned by being whirled about therein in the presence of combustion air. The resultant burning yields the high heat release rates sufficient to maintain a normal means temperature therein above the ash fusion temperature of the fuel. In the operation of this type of cyclone furnace, the ash separates as molten slag from the combustion gases and flows along the bottom of each cyclone furnace into the lower furnace portion 14 and is discharged through the slag openings 21 therein. The combustion products are discharged from the outlet of the cyclone furnace into the unobstructed lower furnace portion 14 whence the gases pass upwardly towards the upper furnace. The spacings between the opposite parallel walls 12 and 16 is wide enough to prevent the combustion products issuing from the cyclone furnaces on one side from carrying over into the cyclone furnaces on the opposite side and an upward velocity is maintained in the lower furnace portion 14 which limits the quantity of slag that is carried into the upper furnace. The entire surface area of the lower furnace chamber 14 is cooled by fluid cooled tubular members covered with refractory to reduce the heat input thereto and to maintain temperatures in the lower furnace portion above the ash fusion temperature so that the molten slag will freely flow out of the slag outlet 21 and into the slag collection chamber 21a. This refractory terminates at a position adjacent the lower end of the gas distributing chambers 50 and 51. The upper furnace chamber portion 26 and the convection chamber B at the top of the setting are cooled by bare tubular members closely spaced and arranged to present percent fluid cooled surface to the flowing gases.

At the juncture between the lower portion 14 and the upper furnace chamber portion 26 recirculated gas from ducts 50 and 51 is injected into the flowing combustion products at a velocity suflicient to cause good mixing upon the subsequent gas expansion caused by the outwardly diverging furnace walls 22 and 23. This expansion process occurs abruptly and changes the velocity into gas turbulence to thus mix the cool recirculated gas products within the hot combustion products. This mixing provides a gas mixture which enters the convection chamber B uniformly mixed and at a controlled temperature depending upon the rate of recirculated gas products. Further, as the upper furnace chamber 26 is very large in volume and is arranged with bare heat absorbing surfaces, the mixed gases are cooled by radiant heat transmission to the walls, and any slag products contained therein solidify and tend to drop on to the walls, due to cooling and/ or turbulence, where the ash is collected in a dry form. The gas leaves the furnace chamber A and passes into the convection chamber B where it passes first over the secondary superheater 28 and subsequently is divided into three controlled streams through the individual passes 30, 31, 32 wherein it simultaneously heats the first reheater 36, the second reheater 37 and the primary superheater 35. Each of the three parallel flow streams is controlled by dampers 41, 42, and 43 which proportion the heat absorption of the reheaters and superheaters as required by the prime mover (not shown). The gas then passes upwardly through the air heater 45 and precipitator 46 to the stack and a portion of the gas is drawn off by the gas recirculation system for further delivery to the furnace as previously described.

With particular reference to FIG. 5 the flow of the vaporizable fluid can best be seen. Water is supplied to the economizer by a feed pump 65 at a pressure of approximately 4500 p.s.i.g. wherein it flows therethrough to become partially heated and cools the gases leaving the setting to an economically low value. The water passes from the economizer to the wall tubes of the cyclone furnaces wherein it is further heated before being passed to the lower water walls. The lower water walls diagrammatically depict the walls of the lower furnace portion 14. The fluid then passes from the radiant lower water walls of the portion 14 into the lower enclosure wall portions which diagrammatically depict the cooling arrangement of the upper portion 26 of the furnace chamber A. The recirculated tempering gas enters the furnace at the juncture of the lower water walls of the furnace portion 14 and the lower enclosure Walls of the upper furnace portion 26. The recirculated gas reduces the temperature of the combustion gases and results in a lower heat input to the enclosure walls where transition occurs. The fluid passes through the transition zone in the lower enclosure walls and, upon reaching the outlet of the lower enclosure walls has already been converted into steam. Thence the steam passes to the support tubes 60 which support a portion of the setting, and after passing through the support tubes the steam is brought back into the boiler setting and passes back into the upper enclosure walls (which line the convection chamber B) before entering a mixing header. In the mixing header the fluid from all of the parallel streams comprising each of the enclosure wall sections is mixed to assure a uniform temperature and then is passed to a primary superheater. After leaving the primary superheater the steam passes through a spray attemperator (preferably of the type shown in the U.S. Patent No. 2,550,683 to Fletcher et a1.) wherein the steam temperature is limited to a predetermined value to assure that a maximum outlet temperature is not exceeded from the secondary superheater. After passing through the attemperator the fluid passes through the secondary superheater to an outlet header 66 before passing to a point in use. Steam after having been partly expanded in the prime mover is returned to the first stage reheater for further heating and is delivered to the outlet header 67 to be returned to an intermediate stage ofthe turbine. Low pressure steam is returned from the turbine, passed through the second stage reheater to the outlet 68 for further expansion in the turbine.

In the schematic drawing in FIG. each portion is shown as a single line. However, in the unit of FIG. 1 each section comprises a multiplicity of parallel flow tubular elements arranged in groups.

The entire setting of the steam generating. unit is enclosed by fluid cooled tubular sections which are arranged in a plurality of adjacent sections from the top to the bottom of the unit. Each of these sections horizontally divides the setting. Thus one section will be on all four walls of the setting at one level and each section comprises a multiplicity of small diameter parallel flowing tubes. Each section is connected to the lower and upper sections for serial flow therethrough, as shown in FIG. 5. Also the connections of the wall cooling sections are such that the cooling fluid rises in temperature as it passes from the lower portion of the setting tothe uppermost section.- Accordingly, the transition zone occurs inthe enclosure walls of the setting and under normal full load operation occurs in the cooling sections that bound the upper portion 26'of the furnace chamber A. It may be further seen that the position of take-off for the tubular support tubes is shown at the connections between two adjacent cooling wall sections. However, the position and consequently the fluid temperature at which the supply to the tubular supports is taken may be varied as hereinafter described.

The control of the steam generation rate of the unit decrioed may be any of the well known types of control. For instance, the transition zone may be kept at a constant position in the unit. Thus the superheater sizewill be fixed, and steam temperature control will be maintained by varying the gas recirculation rate and/ or spray attemperation rate, or steam temperature control maybe maintained by varying the feed rate in direct proportion to 6 the load by allowing the transition zone tomOVe-Withthe load while using gas recirculation to regulate the sla conditions within the furnace.

With special reference to FIGS; 6 and 7there is'shown the apparatus for supporting a portion of the setting by utilizing tension in the tubular supports 60. An outer footing 30 of the cyclone and an inner footing 81 are each supported on a ball bearing supportSZ-which is arranged to transmit the load directly therethrough without a friction load due to transversemovement of thesetting resulting from thermal expansion. The bearings are rigidly mounted on the lower cross beam 61. Thereby, the loads are directly carried by the cross beam 61. A collar 83 is rigidly welded to the tubular member 60. A slideable collar or yoke 84-is loosely placed around the tubular member 60 above the collar 83 and is rigidly'attached to the cross beam member 61. Thus the support arrangement is arranged so that the tubular members carry the load of the cross beam when the downward movement of the cross beam 61 is greater than the downward move.- ment of the tubular member 60. The tubular members 60' contain the fluid at a temperature which is near to the mean temperature for expansionof the entire setting. so that the downward expansion of'the setting is approximately equal to the downward expansion of the tubular elements. The temperature range found most usefulis 600 to 800 F. as the average unittemperature is usually in that range. As shown in FIG. 5 steam just after its transition in the lower enclosure walls of' the furnace portion 26 is passed through the supports. The fluid enters header 62, flows upwardly in alternate tubes and intothe upper header 63. The fluid then passes downwardly in the remaining alternate tubes into the lower outlet header 64 for further passage back to the upper enclosure walls. This arrangement considerably simplifies the support problem of the unit because there does not have to be any provision for absorbing the entire expansion of the unit at operating temperature in the connections to cool supports. Here the supports grow atthe same rate as the unit which it supports and this considerably reduces the problem of making connections because much less allowance and differential thermal expansion must be arranged- The present invention provides a furnace arrangement wherein fuel may be fired above its ash fusion temperature and molten slag collected therefrom in a lower portion of a vertically extending, furnace and recirculated products of combustion may be mixedwiththe hot combustion products then expanded, into a. large radiant chamber lined with bare wall cooling. tubes so that the gases are considerably cooled and controlled in temperature to prevent slagging in a subjacent convection chamber. Further, such an arrangement provides a radiant heat transfer rate in the upper portion of the furnace chamber making: it feasible to allow the transition of once-through boiler to occur therein. Additionally, the furnacechamber and gas recirculation arrangement provides an. extremely simple and uncomplicated structure wherein uniformly mixed gases aredelive'redfto the" convection portion of the. vapor generator.

Additionally, there is provideda plurality of cyclone typefurnaces which are arranged to be fired from opposite parallel walls into. a common unobstructed chamber comprising tubular fluid cooled Walls forming a vertically extending setting of rectangular cross-section providing a furnace chamber and a superjacent convection gas cooling chamber receiving combustion gases flowing upward from said furnace, said furnace chamber having a small volume unobstructed lower portion arranged for the collection of molten slag and a large upper radiant portion arranged with bare metal walls for the cooling of molten slag to dry ash, said furnace walls having a pair of upright fluid cooled parallel opposing walls in said lower portion which diverge upwardly and then run vertical and parallel to form in part a large symmetrical shaped radiant upper portion of said furnace, combustion means providing to said lower portion high temperature products of combustion from slag forming fuel, means introducing low temperature gaseous products of combustion which are taken from a point in said convection cooling chamber into said furnace adjacent the juncture of said lower and upper portions, said fluid cooled walls horizontally divided into a plurality of vertically adjacent cooling sections each arranged for operation with a fluid temperature higher than a subjacent section, each cooling section having a multiplicity of fluid cooling tubes arranged for parallel flow of fluid therein, each of said adjacent wall cooling sections above said lower small volume furnace portion arranged for superheating vapor, and means for flowing high pressure vaporizable fluid once-through said cooling sections in serial flow from the lowest to the highest section of the setting walls.

2. A forced circulation once-through vapor generator comprising tubular fluid cooled walls forming a vertically extending setting of rectangular cross-section providing a furnace chamber and a superjacent convection gas cooling chamber receiving combustion gases flowing upward from said furnace, said furnace chamber having a small volume unobstructed lower portion arranged for the collection of molten slag and a large upper radiant portion arranged with bare metal walls for the cooling of molten slag to dry ash, said furnace walls having a pair of upright fluid cooled parallel opposing walls in said lower portion which diverge upwardly and then run vertical and parallel to form in part a large symmetrical shaped radiant upper portion of said furnace, combustion means providing to said lower portion high temperature products of combustion from slag forming fuel, said combustion means including a plurality of fluid cooled cyclone furnaces arranged on opposite sides of said furnace chamber to separately discharge combustion gases and molten slag through said parallel opposite walls into said lower portion, certain tubes from one pair opposing walls extending inwardly and then upwardly to divide said convection gas cooling chamber into an upper volume having three parallel upflowing gas passes and a lower open volume arranged as a single gas pass and separated from said furnace by said inwardly extending tubes, a primary superheater in one of said parallel gas passes, a first stage vapor reheater in a second of said parallel gas passes, a second stage reheater in the third of said gas passes, a secondary superheater substantially filling the lower single gas pass of said convection cooling gas chamber and arranged to be heated by all of the heating gases, an economizer having three sections with one in each of said three heating gas passes above the pertinent vapor heater, damper means arranged to individually regulate the heating gas flow in each of said three gas passes, and means for providing serial flow of high pressure vaporizable fluid successively through said economizer, cyclone furnaces, lower small volume furnace walls, upper furnace walls, convection chamber walls, primary superheater and secondary superheater.

3. A forced circulation once-through vapor generator comprising tubular fluid cooled walls forming a vertically extending setting of rectangular cross-section pro viding a furnace chamber and a superjacent convection gas cooling chamber receiving combustion gases flowing upward from said furnace, combustion means including a plurality of fluid cooled cyclone furnaces arranged on opposite sides of said furnace chamber to separately discharge combustion gases and molten slag into said lower portion, structural members for top supporting said setting including a plurality of fluid heated vertical tubular support members arranged at spaced positions along the setting to carry in tension a portion of the support load of said setting, a cross beam passing under said cyclone furnaces, ball bearing means on said beam to frictionlessly support said cyclone furnaces, means on said tubular supports for carrying a down loading only of said beam, and means for passing the vaporizable fluid from said setting through said supports at a temperature near that of the average temperature of said setting.

4. A vapor generating unit comprising fluid cooled walls forming a vertically elongated furnace chamber having a small volume unobstructed lower portion arranged for the collection of molten slag and a large upper radiant portion arranged for the cooling of molten slag to dry ash, said furnace walls having a pair of upright fluid cooled parallel opposing walls in said lower portion which diverge upwardly and then run vertical and parallel to form in a part a large symmetrical shaped radiant upper portion of said furnace above the centrally arranged small volume lower portion with the juncture of said lower and upper portions providing an abrupt change in flow area and attendant rapid decrease in gas velocity in said upper portion, combustion means providing to said lower portion high temperature products of combustion from slag forming fuel, said combustion means including a plurality of fluid cooled cyclone furnaces arranged with their central axes substantially horizontal on opposite sides of said furnace chamber to separately discharge combustion gases and molten slag through said parallel opposite walls into said lower portion, and means introducing low temperature gaseous products of combustion which are taken from a point in the vapor generator remote from said furnace into the lower portion of said furnace adjacent the juncture of Said lower and upper portions to provide rapid mixing of the low temperature gases with the high temperature products as they rapidly decrease velocity in said upper chamber.

5. A vapor generating and heating unit adapted to burn a slag producing fuel comprising a setting having front, rear and side fluid cooled walls, said walls enclosing a radiant section and a vertical superposed convection heating section having vapor heating means disposed therein, said radiation section having a lower primary radiation chamber and a superposed enlarged secondary radiation chamber, said front and rear fluid cooled walls adjacent the upper portion of said primary radiation chamber diverging outwardly to form an unobstructed transition section between the upper and lower chambers, a plurality of laterally extending and spaced apart cyclone furnaces disposed in the front and rear walls of said primary radiation chamber, said cyclone furnaces arranged to discharge hot gaseous products of combustion directly into said primary radiation chamber so that gas distribution across the setting is enhanced, said setting being arranged to flow said uniformly distributed gases upwardly therethrough, and means for withdrawing lower temperature heating gas from a position remote from said radiation section and recirculating the same through said radiation section, said gas being introduced into said section below said transition section whereby the recirculated gas and hot combustion gases are mixed as they flow from the lower radiation chamber through the transition section and into the enlarged upper chamber.

6. In a fluid heating unit having a fluid circulation system; walls including fluid heating tubes connected into said system forming an upright gas flow chamber; a cyclone furnace connected to said chamber and supplying high temperature heating gases thereto; means for top supporting said unit comprising means for top supporting said walls, and means for carrying the weight of said cyclone furnace including a support beam disposed thereunder, roll type bearing means on said beam for transmitting the load of said cyclone furnace to said beam and permitting relative horizontal movement between said beam and cyclone furnace, upright tubular support means, means for transmitting the load of and securing said beam to said tubular support means, means for top supporting said upright tubular support means, and means for passing fluid through said tubular support means at a temperature near to that of the average temperature of said fluid heating tubes.

7. A vapor generating unit comprising fluid cooled walls forming a vertically elongated furnace chamber having a small volume unobstructed lower portion arranged for the collection of molten slag and a large upper radiant portion arranged for the cooling of molten slag to dry ash, said furnace walls having a pair of upright fluid cooled parallel opposing walls in said lower portion which diverge upwardly and then run vertical and parallel to form in part a large symmetrical shaped radiant upper portion of said furnace above the centrally arranged small volume lower portion with the juncture of said lower and upper portions providing an abrupt change in flow area and attendant rapid decrease in gas velocity in said upper portion, combustion means providing to said lower portion high temperature products of combustion from slag forming fuel, said combustion means including firing means arranged on opposite sides of said furnace chamber, and means introducing low temperature gaseous products of combustion which are taken from a point in the vapor generator remote from said furnace into the lower portion of said furnace adjacent the juncture of said lower and upper portions to provide rapid mixing of the low temperature gases with the high temperature products as they rapidly decrease in velocity in said upper chamber.

8. A vapor generating unit comprising walls including radiant heat absorbing fluid cooled tubes forming an upright substantially unobstructed furnace, said furnace having a small volume lower portion of substantially uniform horizontal cross-sectional area throughout its height, a large upper portion above said lower portion, and an intermediate portion of substantial height and of continuously increasing horizontal cross-sectional area in the direction of said upper portion opening at its opposite ends to said lower and upper portions, firing means disposed on at least one of the walls of the lower portion of said furnace and directly supplying to said lower furnace portion high temperature products of combustion from slag-forming fuel, a slag outlet from said lower furnace portion, gas recirculation means constructed and arranged to conduct heating gases from a position downstream gas-wise of said furnace to the lower portion of said furnace at a position adjacent the juncture of said lower and intermediate furnace portions for mixing with the freshly generated products of combustion, said intermediate furnace portion being proportioned and arranged and of a height sufficient to provide an abrupt decrease in gas flow velocity between the lower and upper ends thereof, rapid and intimate mixing of the recirculated gases with the freshly generated gases, and uniform distribution of the resulting gas mixture to said upper furnace portion.

References Cited in the file of this patent UNITED STATES PATENTS 1,634,084 Ruths June 28, 1927 2,628,598 Van Brunt Feb. 17, 1953 2,730,080 Stallkamp Jan. 10, 1956 2,774,339 Junkermann Dec. 18, 1956 2,815,007 Sprague et al. Dec. 3, 1957 2,842,105 Kolling July 8, 1958 FOREIGN PATENTS 1,065,655 France Jan. 13, 1954 1,068,954 France Feb. 10, 1954 523,870 Great Britain July 24, 1940 726,244 Great Britain Mar. 16, 1955 727,218 Great Britain Mar. 30, 1955 OTHER REFERENCES Combustion, April 1955, pages 57 to 60.

Steam, by The Babcook & Wilcox Co., 37th ed. of 1955, page 28-6.

Steam, by The Babcock & Wilcox Co., 37th ed., third printing of 1955, pages 28-9; published by Geo. McKibbin & Son. 

